1. Field of the Invention
The present invention relates to a charging apparatus having plural charging means which charge a member to be charged. Such charging apparatus is preferably used in an image forming apparatus for image formation by an electrophotographic process or the like, such as a copying apparatus, a printer or a facsimile machine.
2. Description of Related Art
As shown in FIG. 6, in an image forming apparatus utilizing an electrophotographic process such as a copying machine, a printer, or a facsimile, an electrophotographic photosensitive member 101 (hereinafter called a “photosensitive member”) constituting an image bearing member is uniformly charged by a corona charger (charging device) 102 constituting charging means, and the surface of the charged photosensitive member 101 is subjected to an image exposure L by an exposure apparatus 103 to form an electrostatic latent image. Such electrostatic latent image is developed by a developing sleeve 104a of a developing apparatus 104 to form a toner image, which is transferred by a transfer roller 105 onto a transfer material P such as paper, thereby achieving formation of an image.
Also, residual toner, remaining on the surface of the photosensitive member 101 after the transfer, is removed by a cleaning blade 106, and the photosensitive member 101 is exposed to a pre-exposure lamp 107 to eliminate an optical memory of the image exposure L, thereby being prepared for a next image formation.
As a photosensitive member 101 to be employed in the aforementioned image forming apparatus, an organic photosensitive member and an amorphous silicon photosensitive member (hereinafter represented as “a-Si photosensitive member”) are widely employed, and the a-Si photosensitive member is particularly employed as an electrophotographic photosensitive member for a high-speed copying machine or a laser beam printer (LBP) because it has a high surface hardness, a high sensitivity to a semiconductor laser or the like and scarce deterioration in repeated use.
For charging such a-Si photosensitive members, there are known, for example, a corona charging method utilizing a corona discharge, a roller charging method for charging by a direct discharge utilizing a conductive roller, and an injection charging method of directly injecting a charge into the surface of the photosensitive member, by securing a sufficient contact area for example with magnetic particles.
The corona charging method and the roller charging method mentioned above, utilizing a discharge, tend to cause depositing of a discharge product on the surface of the photosensitive member. In addition, as the discharge product tends to remain on the surface of the photosensitive member because the a-Si photosensitive member has a very high surface hardness and is not easily abraded, the charge on the surface of the photosensitive member bearing an electrostatic latent image moves along the surface thereof, for example by moisture adsorption under a high moisture environment, thereby resulting in an image streaking phenomenon.
On the other hand, in the aforementioned injection charging method, based on direct charge injection from a part in contact with the surface of the photosensitive member, the aforementioned image streaking phenomenon is less likely to appear.
In the a-Si photosensitive member, which is prepared by turning a gas into a plasma by a high frequency current or a microwave and depositing the gas as a solid film on an aluminum cylinder, the film thickness or the composition becomes uneven in the circumferential direction unless the plasma is generated uniformly.
Also in the image exposure of the charged photosensitive member in the exposure apparatus, a potential on the photosensitive member is attenuated (lowered) in an exposed portion (light area), and, in the a-Si photosensitive member, a potential attenuation becomes much larger even in a dark area (non-exposed portion) in comparison with an organic photosensitive member and also increases by an optical memory effect of the image exposure, so that it is preferable to provide pre-exposure means in front of the charging, in order to erase the optical memory of the preceding cycle.
For this reason, there results a very large potential attenuation, about 100 to 200 V, between a charging position and a developing position on the photosensitive member along the rotating direction thereof. In addition, because of the aforementioned unevenness in the film thickness of the a-Si photosensitive member, there results a potential unevenness of about 10 to 20 V along the circumferential direction of the photosensitive member.
In particular, the a-Si photosensitive member, having a larger electrostatic capacitance in comparison with the organic photosensitive member, is more strongly influenced by such potential unevenness, and shows an unevenness in the image density more conspicuously.
Against such drawback, it is effective to employ for example a method of charging plural times. In such method of such plural chargings against the aforementioned increase in the potential attenuation in the dark area resulting from the optical memory effect, since the optical memory can be significantly reduced in a first charging, the potential attenuation in the dark area can be reduced after a second charging. It is therefore possible to significantly alleviate a potential ghost phenomenon or a potential unevenness.
While the charging with plural injection charging devices can significantly alleviate the potential ghost phenomenon or the potential unevenness, it tends to enhance the abrasion of the surface of the photosensitive member, since a greater number of contact points with the photosensitive member are required for charge injection, for example, in case of a magnetic brush charger, since a magnetic particle carrying member is moved opposite to the moving direction of the photosensitive member to cause a frictional motion of the magnetic particles.
The a-Si photosensitive member, having a very hard surface, can provide a certain endurance even in case of using plural magnetic brush chargers, but there is desired a higher endurance because the photosensitive member is associated with a high cost because of the manufacturing process thereof. On the other hand, in case of charging the a-Si photosensitive member with a plural charging system employing corona chargers or roller chargers based on discharges, there can be obtained a high endurance against the abrasion but an image streak phenomenon tends to appear because due to depositing of a discharge product, as explained above.
Also in an a-Si photosensitive member, in order to employ plural charging means for alleviating the potential ghost phenomenon and the potential unevenness, and to achieve an improvement in the abrasion resistance and a prevention of the image streak at the same time, it is effective to combine the aforementioned injection charging method and a method of lower friction such as a corona charging method or a roller charging method.
Such combination of the injection charging method and the corona charging method or the roller charging method allows for elimination of the discharge product, generated by the corona charging or the roller charging, by the friction of the injection charger, and to maintain the abrasion of the photosensitive member almost at a level of a case of employing an injection charger only, by reducing a coating amount of the magnetic particles on the magnetic particle carrying (bearing) member, thereby decreasing the frictional force.
Also, even when employing plural magnetic brush chargers, it may be possible to realize an acceptable drum service life by reducing the coating amount of the magnetic particles, thereby decreasing the frictional force.
However, a reduction in the coating amount of the magnetic particles for decreasing the frictional force results in a decrease of the charging ability of the magnetic particle carrying member, eventually leading to a conspicuous unevenness in the charging ability. Also, other charging means, for example the roller charger, result in an unevenness in the charging ability in case toner or dust is deposited on the roller surface.
In particular, in case an unevenness in the charging ability results in the lowermost charging means (most downstream side) in the moving direction of the photosensitive member, it becomes difficult to apply a uniform charge on the surface of the photosensitive member, thereby causing a significant influence on the output image.